In analytical chemistry, liquid chromatography (LC) and gas chromatography (GC) techniques have become important tools in the identification of chemical sample components. The basic principle underlying all chromatographic techniques is the separation of a sample chemical mixture into individual components by transporting the mixture in a carrier fluid through a porous retentive media. The carrier fluid is referred to as the mobile phase and the retentive media is referred to as the stationary phase. The principal difference between liquid and gas chromatography is that the mobile phase is either a liquid or a gas, respectively.
In a GC apparatus, an inert carder gas typically is passed through a temperature-controlled column which contains a stationary phase in the form of porous sorptive media. Gas chromatography columns have also been known to comprise a hollow capillary tube having an inner diameter in the range of few hundred microns coated with the stationary phase. A sample of the subject mixture is injected through an inlet into the carrier gas stream and passed through the column. As the subject mixture passes through the column, it separates into its various components. Separation is due primarily to differences in the partial pressures of each sample component in the stationary phase versus the mobile phase. These differences are a function of the temperature within the column. A detector, positioned at the outlet end of the column, detects each of the separated components contained in the carrier fluid as they exit the column.
Inlet pressure and flow setpoints for chromatographic analysis which are controlled electronically by a pneumatic closed-loop controller offer increased precision and ease of use because there is a large dynamic range of use in terms of pressure/flow combinations. This large dynamic range of applications has a substantial effect on the natural response of the pneumatic system with respect to changes in the drive to the electronic valve used by the electronic controller to change inlet pressures or flows. The pneumatic response (pressure or flow) to a change in valve drive can be considered as a transfer function. This response is a function of frequency and is termed its "frequency response". In addition to pressure and flow, other variables within the inlet also have significant effects on the frequency response of the inlet pneumatics, which is described in terms of gain and bandwidth. These variables include tank pressure (which primarily effects gain), the gas type (which effects gain and bandwidth), the liner type and packing (which effects gain and bandwidth), the presence and type of chemical trap in the split line (effects gain and bandwidth), and the "flow gain" of the proportional valve (which primarily effects gain). This large dynamic range and many interdependent variables present a problem for the designer of the proportional integral derivative (PID) compensation loops for the inlet pressure and flow controllers. If the PID coefficients are optimized for stability with the worst case conditions, the performance is sluggish for customers who do not use the inlet under these worst case conditions. Tuning the PID loops to control the inlet for the "typical cases" may result in unstable operation for analyses operated at the fringes of "user space" or where the customer selected gas types, liner types, chemical traps, etc. result in the worst case conditions for stability. As users change the inlet configuration, the pressure control of the inlet can begin to oscillate.
For any pneumatic system, when flow is increased, the "gain" of the inlet increases, and that as the pressure of the inlet is increased, the "bandwidth" of the inlet increases. Both of these terms significantly effect the control coefficients for the electronic control loop of the inlet pneumatics. For example, if the proportional integral derivative (PID) controller gain is too large, at high inlet flows (i.e. high "gain" conditions), the inlet flows may oscillate due to too much overall gain. Correspondingly, if the PID compensation is properly "tuned" for a high pressure case, and the pressure setpoint is changed to a low pressure (i.e. "bandwidth" conditions), the pressure control loop could oscillate due to too much phase shift within the system.
FIG. 1 illustrates a functional block diagram of the pneumatic system solution offered by Fisons. The inlet has only one electronic control loop, either pressure or flow, and a mechanical pressure regulator across one of two restrictors (R1, R2) for controlling split vent flow. The system has a single electronic control loop that controls either pressure or flow and lacks programmable flexibility. It can not compensate for the effect of one variable on the other (e.g. high "gain" conditions caused by high flow setpoints can not be used to compensate a pressure control loop). Furthermore, there are a large number of components in the sample path. Not only does each component shrink the available user space, each component adds a variable to the transfer function can effect the stability of the single electronic control loop.
Another technique for electronic control is shown in FIG. 2, a functional block diagram of the Shimadzu 17A, a split/splitless inlet gas chromatograph, manufactured by Shimadzu. A mass flow controller is connected at the input to the inlet and a back pressure regulator is connected at the output of the inlet. A buffer and packed tubing are included in this system. Although the control loops are programmable, the addition of the buffer and packing tubing reduces the bandwidth and thus increases the response time of the inlet to setpoint changes or disturbances rejection (such as the pressure "pulse" that happens during vaporization of the injected solvent).
It would be beneficial to characterize the pneumatic system of a gas chromatograph as a transfer function in which all of the variables are described in terms of either pressure or flow. It would be an additional benefit if the transfer function were used to alter the electronic control of the pneumatic system to improve the stability of the system and the response time to setpoint changes over a broad range of user pressure and flow setpoints.